1. Field of the Invention
The present invention relates to an internal combustion engine in which it is possible to set a period during which both an intake valve and an exhaust valve are closed from an exhaust stroke to an intake stroke.
2. Description of the Related Art
As is well known in the art, generally speaking, in a four-cycle internal combustion engine, an improvement in theoretical thermal efficiency is achieved through an increase in the ratio of specific heat of a working fluid by making an air fuel mixture lean. However, in the case of a gasoline engine, in which spark ignition is effected from the viewpoint of controlling the ignition timing, the flame core formation by ignition and flame propagation are rather difficult to perform, which leads to a problem in ignitability and limitations in the leaning of the air fuel mixture. As in the case of a gasoline engine, in the case of a gas engine using natural gas, propane gas or the like, ignition is rather difficult to effect, resulting in a problem similar to that in a gasoline engine. In the case of a diesel engine using light oil, the ignitability is satisfactory and thus self-ignition is performed, and substantial leaning is possible. However, it is liable to generate soot, and involves a problem regarding NOx discharge. Thus, from the viewpoint of increasing thermal efficiency and reducing NOx discharge, an improvement in ignitability is desired even in the case of a fuel not to be ignited easily, such as gasoline or natural gas.
From the viewpoint of promoting ignitability, an internal combustion engine is known which is equipped with a variable valve timing mechanism for adjusting the opening/closing timing for the intake and exhaust valves and in which it is possible to provide a period during which both the intake and exhaust valves remain closed from an exhaust stroke to an intake stroke (See, for example, JP 2000-64863 A (at pages 2–5, FIG. 4) and JP 2001-355462 A (at pages 2–3, and 7, and FIG. 8)). According to JP 2000-64863 A, under high load condition, an air fuel mixture is ignited by an ignition device, whereas, under low load condition, the timing for closing the exhaust valve is set such that advancement is effected during the exhaust stroke before the piston top dead center. Further, JP 2001-355462 A also discloses a technique in which the closing of the exhaust valve is controlled so as to be effected on the advance side with respect to the intake top dead center. By thus providing the period during which both the intake and exhaust valves remain closed on the advance side with respect to the intake top dead center, residual gas is trapped and newly sucked air is heated, thereby promoting ignition.
In the conventional technique described above, the ignitability depends on the heating effect of the residual gas trapped, so that, to promote the ignitability, it is necessary to augment the amount of residual gas trapped. However, when the amount of residual gas trapped increases, the heat loss and pump loss due to the compression and expansion of the gas increase, resulting in a deterioration in thermal efficiency. Further, since the intake amount of the newly sucked gas decreases, the load range allowing operation is restricted.